As a machining method using a machining device, such as a robot, an incremental forming method has been known in which a workpiece is machined by attaching a bar-like tool (machining jig) to a movable part, such as a robotic arm, and moving a tip-end part of the tool relatively with respect to the workpiece while pressing the tip-end part of the tool against the workpiece (e.g., see Patent Document 1). By using such an incremental forming method, a plate-like workpiece can be fabricated with a high degree of freedom without using a die.